1. Field of the Invention
The present invention relates to an antenna device of an in-vehicle device that is used in a communication system for performing unlock/lock or the like of a vehicle door between an in-vehicle device mounted at the vehicle and a portable device carried with a user. More specifically, the present invention relates to an antenna device that forms an arrival range (hereinafter, referred to as a communication range) of a transmission request signal that is transmitted in order to detect the existence of the portable device.
2. Description of the Related Art
Recently, there is popularized so-called, a smart entry system for performing unlock and lock or the like of a vehicle door only when a user approaches the vehicle or departs from the vehicle while carrying a potable device. Because the smart entry system can unlock and lock the vehicle door without a mechanical key, it is excellent in convenience.
According to this system, the in-vehicle device mounted at the vehicle outputs a transmission request signal through an antenna device. The portable device that receives this transmission request signal sends a reply signal to the in-vehicle device. The in-vehicle device that receives the reply signal controls a door actuator to unlock and lock the vehicle door.
The above-mentioned in-vehicle device is provided with a plurality of antenna devices. The antenna devices include:
an antenna device having a transmission antenna for an outside of the vehicle that is disposed at a transmitting unit and, for example, in a door handle of each vehicle door; and
an antenna device having a transmission antenna for an inside of the door that is disposed in the vicinity of the transmitting unit and, for example, an instrument panel.
The transmitting unit is driven by a control unit of the in-vehicle device in the antenna device. The transmitting unit outputs the transmission request signal to a predetermined communication range through the transmission antenna.
Formation of the communication range in a conventional antenna device used in this system will be demonstrated with reference to FIG. 8 and FIG. 9.
FIG. 8 is a block diagram of the conventional antenna device. FIG. 9 is waveform diagrams demonstrating an operation of the conventional antenna device.
Referring to FIG. 8, in transmitting unit 51 of antenna device 50, binary signal Sa is input from a control unit of an in-vehicle device (not shown) to modulation unit 52 formed with an AND circuit through input terminal 56, and carrier signal Sb is input from the control unit of the in-vehicle device to modulation unit 52 through input terminal 54. Binary signal Sa is a signal having a duty ratio of 50% that repeats High (H)/Low (L) shown in FIG. 9. Carrier signal Sb is a carrier signal that forms a pulse string shown in FIG. 9. Modulation unit 52 modulates carrier signal Sb by binary signal Sa and outputs modulated signal Sf shown in FIG. 9.
In FIG. 8, driving circuit 57 is formed with connecting in series a pair of power transistors between power supply Vd and earth (GND). First power transistor 121 on power supply Vd side is P channel FET, and second power transistor 122 on the GND side is N channel FET. Moreover, first power transistor 121 and second power transistor 122 are provided with parasitic diodes 121a and 122a in parallel, respectively.
Modulated signal Sf is input from modulation unit 52 to first power transistor 121 and second power transistor 122 of driving circuit 57, respectively.
In FIG. 8, transmission antenna 55 is formed so that coil 55a and capacitor 55b is connected to each other in series. One end of transmission antenna 55 is connected to a middle point 124 between first power transistor 121 and second power transistor 122 through wiring 152, terminal 58, and resistance 53 which is disposed at transmitting unit 51. The other end of transmission antenna 55 is connected to GND on the circuit side through wiring 154 and terminal 59. That is, transmission antenna 55 is connected to second power transistor 122 in parallel.
Resistance value Ra of resistance 53, inductance La of coil 55a and capacitance Ca of capacitor 55b are referred to as antenna constants. Transmission antenna 55 has Q factor indicating strength of a prescribed resonance that is decided by the antenna constant. This Q factor is proportional to La/Ra of the antenna constant, and when the value of La is made constant, it has the characteristic of Q∝1/Ra. Generally, it is performed to reduce a winding number of a coil and to form the transmission antenna in order to cheapen transmission antenna 55. The Q factor of the conventional art transmission antenna 55 is relatively small, for instance, Q=10.
Antenna device 50 is configured such that transmission antenna 55 is connected to transmitting unit 51 as described above.
According to the above-mentioned configuration, modulation unit 52 controls ON/OFF state of driving circuit 57 by modulated signal Sf in antenna device 50. As a result, antenna current Ie shown in FIG. 9 flows to transmission antenna 55. Transmission antenna 55 transmits intensity of the transmission request signal according to antenna current Ie and forms the communication range that is substantially in proportion to the size of antenna current Ie.
That is, in t1 (t-ON) period (during energizing) where binary signal Sa is H and modulated signal Sf repeats H/L, modulation unit 52 alternately controls ON/OFF state of first power transistor 121 and second power transistor 122. For this reason, transmission antenna 55 becomes in the energizing state. At this time, as shown in the waveform of positive polarity envelope of FIG. 9, since Q factor of transmission antenna 55 is Q=10 which is relatively small, antenna current Ie becomes energizing current 91 that is saturated to the maximum current soon after rising.
In t2 (t-OFF) period (during non-energizing the current) where binary signal Sa is L and modulated signal Sf is also L, modulation unit 52 controls only power transistor 122 at ON state. For this reason, transmission antenna 55 becomes in the non-energizing state. At this time, antenna current Ie is consumed by resistance 53 and becomes non-energizing current 92 that converges to zero soon after falling.
As described above, since Q factor of transmission antenna 55 is small in any case of the energizing current 91 and the non-energizing current 92, antenna current Ie of transmission antenna 55 has the characteristic that is immediately saturated or converged. In antenna device 50, energizing current 91 is changed by varying resistance Ra of the antenna constant, and the communication range that is substantially in proportion to the maximum value is formed.
That is, in antenna device 50, since the maximum value of the energizing current 91 flowing into transmission antenna 55 is changed by resistance Ra of the antenna constant, as shown in FIG. 9, large energizing current J1 flows into transmission antenna 55, when R is small. Moreover, small energizing current J2 flows into transmission antenna 55, when R is large. For this reason, for example, the desired communication range is formed at the inside or outside of the vehicle in proportion to the size of the energizing current 91 that flows into each transmission antenna 55 through transmission antenna 55 arranged in the door handle or the vicinity of the instrument panel.
For example, Japanese Patent Unexamined Publication No. 2002-47835 is known as information of a conventional art document that relates to the above-mentioned technology.
According to the conventional art antenna device as described above, the formation of the communication range is performed with varying resistance value Ra in the resistance of the antenna device. Accordingly, the individual communication range, which differs depending on the arrangement position of the transmission antenna, vehicle model or the like, is set by varying resistance Ra of each antenna device.
It is complicate to set the communication range by varying this resistance value Ra. That is, every time the communication range is measured by using an experiment vehicle or the like, operation that attaches again resistance with soldering iron is accompanied. Furthermore, the communication range is changed when the arrangement position of the transmission antenna or the vehicle design etc. are varied between from the experiment vehicle to a finished vehicle. Therefore, similar operation is performed in each case of those changes.
An universal article is generally used as the resistance. The resistance value is decided within the range of, for example, 5Ω to 12Ω, and the range is changed gradually into 4.9 Ω, 5.6 Ω, 6.8Ω, . . . , according to JIS standard or the like. Therefore, the formation of the communication range is difficult when such a resistance as 5.3Ω that is not included in the JIS standard is necessary. Accordingly, the formation of the communication range with a good accuracy is difficult.